In frequency shift key (FSK) modulators, a carrier wave is shifted in frequency by discrete amounts in response to discrete input signals. Typically, prior art FSK modulators include a source for deriving a control input of a voltage controlled oscillator or a phase locked loop. It is desirable for an output signal of an FSK modulator to be phase coherent and stable. However, considerable circuitry is required in the prior art modulators to provide an FSK modulator output that is both phase coherent and stable. In addition, prior art phase locked loops have considerable settling times which must be considered. With existing phase locked loops, the amount of time necessary to change from one frequency to another is frequently relatively long.
It is, therefore, an object of the present invention to provide a new and improved frequency shift key modulator.
Another object of the present invention is to provide a new and improved frequency shift key modulator which achieves phase coherence and stability with few parts and relatively simple circuitry.
An additional object of the present invention is to provide a new and improved FSK phase coherent and stable modulator capable of instantaneously changing a carrier wave frequency.